Cigarette smoking is a prevalent, modifiable risk factor for increased morbidity and mortality in the United States, and perhaps in the world. Smokers incur medical risks attributable to direct inhalation. Bystanders, termed passive smokers, also incur medical risks from second-hand smoke. Society, as a whole, also bears the economic costs associated with death and disease attributable to smoking. Although the majority of smokers have tried repeatedly to quit smoking, eighty percent of smokers return to tobacco in less than two years after quitting. Therefore, tobacco dependence is a health hazard for millions of Americans.
Nicotine is the biologically active substance that is thought to promote the use of tobacco products by approximately one-quarter of the world populations. Tobacco-related disease is personally and economically costly to the any nation. Unfortunately, once use of tobacco has begun, it is hard for a smoker to quit because of nicotinic dependence and addiction.
The initiation and maintenance of tobacco dependence in a human is due to certain bio-behavioral and neuromolecular mechanisms. Nicotinic acetylcholine receptors (nAChRs) in humans are the initial binding sites for nicotine. The binding of nicotine to nAChRs is thought to modulate the brain's “reward” function by triggering dopamine release in the human brain. The nAChRs exist as a diverse family of molecules composed of different combinations of subunits derived from at least sixteen genes. nAChRs are prototypical members of the ligand-gated ion channel superfamily of neurotransmitter receptors. nAChRs represent both classical and contemporary models for the establishment of concepts pertaining to mechanisms of drug action, synaptic transmission, and structure and function of transmembrane signaling molecules.
Basic cellular mechanisms of nicotinic dependence also involve the functional state changes during repeated nicotinic agonists exposure and receptor changes in the number of receptors during chronic nicotinic exposure. nAChRs can exist in many different functional states, such as resting, activated, desensitized or inactivated The activation and/or desensitization of nAChRs plays an important role in initiating nicotinic tolerance and dependence. Recovery from receptor activation and/or desensitization contributes to nicotinic withdrawal symptoms.
The most abundant form of nAChRs in the brain contains α4 and β2 subunits. α4β2-nAChRs bind nicotine with high affinity and respond to levels of nicotine found in the plasma of smokers. α4β2-nAChR also have been implicated in nicotine self-administration, reward, and dependence. Therefore, selective drug action at nAChRs, especially at those containing α4 subunits, is thought to be an ideal way for nicotine cessation and reducing nicotine withdrawal syndrome. Unfortunately, thus far, no optimal compound can meet this purpose. The brain-blood-barrier permeable nAChR antagonist, mecamylamine is popularly used systemically but exhibits much less nAChR subtype selectivity.
Although a variety of psychopharmacological effects contribute to drug reinforcement, actions on the mesolimbic dopaminergic pathway is the predominant hypothesis for mechanisms of nicotinic reward. The mesolimbic dopaminergic pathway originates in the ventral tegmental area (VTA) of the midbrain and projects to forebrain structures including the prefrontal cortex and to limbic areas such as the olfactory tubercle, the amygdala, the septal region, and the nucleus accumbens. Many studies have indicated that dopamine release in the nucleus accumbens of the human brain is “rewarding” or signals an encounter with a “reward” from the environment. Other substances, such as alcohol, cocaine, and opiates, operate in the same manner, resulting in a cycle of substance or alcohol abuse.
Therefore, a considerable need exists for a novel compound that can selectively block α4 subtypes of nAChRs to prevent smoking-induced “reward”, to limit increasing nicotine-induced dopamine release, and/or to diminish nicotinic withdrawal symptoms.